Field of the Invention
The present invention relates to compositions for inhibiting scale, and relates more particularly to compositions for extending the lifetime of a scale inhibitor treatment in a subterranean formation.
Background of the Related Art
The scale inhibitor squeeze treatment is a well-known method that is widely used in the oilfield to prevent mineral scale formation. Squeeze lifetimes are dependent on a number of factors. Some of these factors are reservoir temperature, brine composition, reservoir mineralogy, and scale inhibitor chemistry. In an attempt to reduce the number of squeeze treatments in a given well, and thereby simplify the logistics and improve the economics of the treatment, modifications to the treatment procedures have been developed that are targeted at extending squeeze lifetimes. The improved performance of scale inhibitor squeeze treatments, coupled with increased squeeze lifetimes, is especially important for critical applications in environmentally sensitive areas.
In situ adjustments to the reservoir brine pH, supplementation with calcium or zinc ions, addition of microcrystalline kaolin, and the incorporation of chemical bridging agents have been used with varying degrees of success. These methods, while offering a degree of squeeze life enhancement, are not without their drawbacks. Methods involving pH adjustment or calcium ion supplementation create the opportunity for damage caused by excessive precipitation of solids. The injection of microcrystalline kaolin can be troublesome in that solids are intentionally introduced into the formation and could lead to plugging of pore throats within the porous medium of the targeted subterranean petroliferous formation. Many chemical bridging agents have temperature limitations restricting their use to a few select reservoirs or have undesirable environmental profiles.
The scale inhibitor squeeze treatment has long been an important tool in the efficient prevention of mineral scale deposition in the surface hardware, in the wellbore and in the reservoir. Once completed, the scale inhibitor squeeze requires no regular maintenance, other than the occasional measurement of residual chemical. Scale squeeze treatments can provide protection from mineral deposition for months after the treatment and, in some cases, even years after the treatment.
Scale squeeze life extension has been an important area of investigation for a number of reasons. Fewer squeeze treatments result in less money being spent on chemicals and the associated costs of the manpower and equipment needed to apply the treatment. Fewer days out of service means that the well continues producing oil or gas. Performing fewer squeeze treatments may also mean that less chemical is released into the environment and there are fewer opportunities for spills and other accidents.
The scale squeeze injection typically comprises three steps in its application. The first step is a Spearhead, alternatively referred to as a Preflush, that prepares the wellbore and reservoir for the treatment. The second step is a Scale Inhibitor Pill, alternatively referred to as a Main Pill, which contains the scale inhibitor. The third step is an Overflush, which distributes the scale inhibitor over a larger volume of reservoir.
For aqueous scale squeeze treatments, the Spearhead fluid may be, for example, clean, filtered, produced brine, or brine prepared by adding inorganic salts, such as KCl or NH4Cl to water, or filtered seawater. The Spearhead fluid may also contain one or more additives, such as surfactants, emulsion preventers, mutual solvents, oxygen scavengers, corrosion inhibitors or biocides, and in some cases Squeeze Life Extension agents. These additives prepare and condition the wellbore and reservoir for the subsequent Scale Inhibitor Pill that contains the scale inhibitor.
The Spearhead and Scale Inhibitor Pill are typically prepared by what is known as batch mixing. For the Spearhead, the entire volume of brine is contained in a portable tank and the Preflush additives are mixed into the brine. Similarly, the entire volume of brine for the Scale Inhibitor Pill is contained in a separate portable tank, and the scale inhibitor(s) are added into it and mixed well. A suitable pump is then used to sequentially inject the fluids into the wellbore. In some cases, the Spearhead and Main Pills are prepared “on the fly”, whereby brine is pumped from a source, the chemical additives are injected into the flowing brine, and the chemical additives are mixed into the brine by means of a static mixer or other mechanical device before entering the wellbore.